The present invention relates to an intake system for an internal combustion engine which may considerably facilitate the evaporation and atomization of the fuel charged into the intake system.
In general, the conventional intake systems for internal combustion engines comprise a fuel supply system including a carburetor or a fuel injection valve, a butterfly type throttle valve disposed within an intake pipe and an intake manifold for distributing the air-fuel mixture into cylinders of the engine.
With the intake systems of the type described above, uneven or nonuniform distribution or charge of the air-fuel mixture into the engine cylinders tends to occur very frequently, resulting in the decrease in engine performance and in the increase of pollutant emission. Such problems are mainly caused by the fact that the fuel which adheres to the wall surface of the intake pipe flows in the form of film toward the respective cylinders so that the air-fuel ratio of the charge drawn varies from one cylinder to another. The adhesion of the fuel to the walls of the intake pipe in turn is caused by the fact that the fuel has not been atomized satisfactorily.
Especially in the intake systems of the type wherein the fuel injection valve is disposed at the upstream of the throttle valve, the fuel injection pressure is considerably high positive pressure so that the fuel is injected and sprayed independently of the conditions of the air flow. As a result, even when the fuel is sprayed uniformly in the intake pipe, a lean mixture is produced at the portions where the air flows at high velocities while a rich mixture is produced at the portions where the air flows at low velocities. Consequently, the air-fuel ratios of the charges drawn into the respective cylinders vary widely from one cylinder to another.